Anti-lock braking systems (“ABS”) are used to increase traction and control of a vehicle under difficult braking conditions. On slippery surfaces, braking can cause the wheel to “lock up” and stop rotating. As a result, the portion of the wheel in contact with the road simply slides relative to the surface, which severely reduces traction and the ability to steer the vehicle. The loss (reduction) of traction causes the truck or tractor-trailer to become unstable and directional control becomes difficult. This problem is of particular concern for heavy commercial vehicles, such as tractor-trailers, where the loss of control can cause the vehicle to jack-knife.
ABS systems prevent wheel lock up by rapidly releasing and reapplying pressure to the brakes, to permit the skidding wheel to regain traction and steering. ABS systems typically comprise a speed sensor positioned adjacent to four or more wheels of the vehicle. Each speed sensor is connected to a controller that monitors the acceleration/deceleration of the wheels. When wheel lock up occurs, the controller detects the abnormally abrupt deceleration of the wheel and operates a valve in the corresponding brake line to release pressure on the brake until the wheel begins to accelerate again. Once the controller senses the wheel accelerate, the controller reverses the valve to allow brake pressure to increase once more. The controller repeats this cycle of acceleration/deceleration many times per second, such that braking is kept near, but does not exceed, the point at which the wheel locks up.
Speed sensors typically operate by detecting the rate of rotation of the wheel hub, brake drum or brake rotor. In the case of an ABS brake rotor, evenly spaced teeth are integrally cast on the surface of the brake rotor in the shape of a ring. The ring of teeth is commonly referred to as a tone ring or exciter ring, and the teeth are referred to as pulse teeth or exciter teeth. A typical exciter ring has about 80 to 120 teeth, depending on the size of the tire. The sensor typically comprises a magnetic pickup that is positioned adjacent to the exciter ring and measures the speed of the rotor by detecting the change in magnetic flux caused by the movement of the exciter teeth.
A variety of methods are used to detect changes in magnetic flux, including variable reluctance, Hall effect and magnetic resistance, as are well known in the art. In each case, the strength of the signal received by the speed sensor decreases with distance from the exciter ring. Accordingly, some skill and care must be used to ensure that the speed sensor is positioned as close as possible to the exciter ring without actually contacting the exciter teeth.
ABS brake rotors also require additional steps of machining and inspection to ensure that the exciter teeth are uniform and that the exciter ring is not damaged. Brake rotors are typically cast in rough form and then machined to their finished dimensions. However, the numerous teeth of the exciter ring are difficult to cast and frequently have casting defects and imperfections which require the rotor to be scrapped. In addition, the exciter teeth are susceptible to damage caused by mishandling following the casting step of the manufacturing process, which increases the scrap rate. Furthermore, the exciter teeth must have uniform depth and clean surfaces for the speed sensors to function properly. Thus, the exciter ring requires an additional machining step to remove any imperfections that would interfere with the operation of the speed sensor. Further yet, the exciter ring is integral with the brake rotor and unrepairable damage to the exciter ring results in discarding of the entire brake rotor. Replacing entire brake rotors due to damaged exciter rings can become very expensive. In addition, dirt and other debris accumulates on the exciter ring, which increases the rate of corrosion. As a result, the exciter ring is frequently the limiting factor in the life of a brake rotor.
Thus, it would be desirable to provide an exciter ring that is formed separately from an ABS brake rotor and is removably connectable to the brake rotor to facilitate relatively easy replacement of the exciter ring when the exciter ring becomes damaged. In addition, it would be desirable to provide a simple, inexpensive, and quick process of refurbishing a wheel assembly when the exciter ring or exciter teeth are damaged.